According to a second aspect, the present invention relates to a field emission light source comprising the anode.

BACKGROUND OF INVENTION In US 5,877, 588 a light source is disclosed that includes an evacuated container having walls at least a portion of which consists of an outer glass layer on which at least a major part thereof is coated on the inside with a layer of phosphor forming a luminescent layer. A conductive layer form is an anode. The layer of phosphor is excited to luminescence by electron bombardment from a field emission cathode located in the interior of the container, and a modulator electrode or grid is arranged between the cathode and the anode for creating an electric field for the emission of electrons.

The field emission cathode includes field-emitting bodies in the form of fibres, and a base body having a longitudinally extending core formed by at least two wires between which the fibres are secured.

In US 6,008, 575, a light source is presented that comprises an evacuated container having walls, at least a portion of which consists of an outer glass layer which on at least a major part thereof is coated on the inside with a layer of phosphor forming a luminescent layer and

a conductive layer forming an anode. The layer of phosphor is excited to luminescence by electron bombardment from a field emission cathode located in an interior of the container. A modulator electrode is arranged between the cathode and the anode for creating an electrical field necessary for the emission of electrons. The phosphor layer is a luminescent layer which upon electron bombardment emits visible light. The anode is preferably made of a reflecting, electrically conductive material, e. g. aluminium.

In this art, there is a number of limitations of the field emission light sources disclosed above. For instance, a field emission light source of the above type may present energy losses in the electrically conductive material due to absorption. There is also a risk of accumulation of charge. In addition, the manufacture of light sources of the above type presents a rather high complexity.

SUMMARY OF INVENTION Key objects of two aspects of the present invention are to alleviate the drawbacks in the art.

According to a first aspect of the present invention, an anode in a field emission light source is disclosed. The anode comprises an electrically conductive layer and a luminescent layer that is luminescent when excited by electron bombardment caused by a potential difference between the electrically conductive layer and a cathode. Key features of this aspect are that the luminescent layer is arranged between the electrically conductive layer and the cathode, that the electrically conductive layer is a transparent electrically conducting layer, and that the thickness of the transparent

electrically conducting layer is in the interval 100- 1000nm.

The transparent electrically conducting layer offers the opportunity of more electrons being excited in the luminescent layer since the electrons does not have to go through the transparent electrically conducting layer before they excite the luminescent layer. The transparent electrically conducting layer being transparent is a necessity since otherwise no light would be transmitted through it. The conductance of the transparent electrically conducting layer will be proportional to the thickness in the case of bulk conduction. However, the conduction mechanism undergoes transitions from insulating type, when the average thickness of the transparent electrically conducting layer is so small that it consists of detached crystallites, to percolation type, when the average thickness is still small, the transparent electrically conducting layer consists of weakly coupled crystallites, and finally to bulk conduction, when the thickness is sufficiently large, the transparent electrically conducting layer has become a continuous film. Bulk conduction usually emerges, at the average thickness above 100 nm. The relation between the thickness and the transparency of the electrically conducting layer is such that the transparency decreases as the thickness increases. As the thickness increases above 1000nm, the transparency becomes such that a sufficent amount of light would not transmitted through it. Based on these considerations, the thickness should be below 1000 nm.

In determining optimum thickness of the layer factors such as heat capacity should also be taken into account.

In a preferred embodiment, the luminescent layer is directly adjacent to the transparent electrically conductive layer. This offers the advantage of further decreasing energy losses of the electrons.

In another preferred embodiment, the thickness of the transparent electrically conducting layer is in the interval 300-700nm, more preferably is in the interval 400-600nm and even more preferably is in the interval 450-550nm. When the thickness of the electrically conducting layer is in the vicinity of 500nm, a favourable relation between the conductance and the transparency occurs, with excellent production economy and performance characteristics.

In another preferred embodiment, the potential difference is in the interval of 4-12 kV, and more preferably is in the interval 5-llkV. A potential difference within this interval offers the advantage of providing a suitable electron bombardment for exciting the luminescent layer to emit light.

In another preferred embodiment, the transparent electrically conducting layer is at least one of: Indium Tin oxide (ITO), and Zinc oxide (ZnO). These materials offer the advantage that they are transparent and present suitable conductive properties.

In another preferred embodiment, the anode further comprises an enclosing transparent structure on which the electrically conducting layer is fixed.

In another preferred embodiment, the transparent structure is a made of glass.

According to a second aspect of the present invention, a field emission light source comprising the anode according to the first aspect is disclosed.

In terms of enabling the present invention, a number of alternatives manufacturing processes are available.

For instance, the anode may be manufactured by using a pouring in and pouring out process, a spray method, spin coating or printing.

At this stage it is important to make a distinction between two physical principles regarding field emission light sources, namely cathodoluminescence and photoluminescence. The physical principle behind the lighting process concerned in the present invention is cathodoluminescence, wherein the production of visible light is done by direct impingement of electrons on the phosphors. Photoluminescence, on the other hand, concerns a fluorescent light source based on a phosphor excited by a molecular discharge. In a fluorescent light source, plasma is produced from atomic or molecular vapour in the lamp enclosing transparent structure. The radiation produced by the plasma is used to excite a phosphor coated on the inner surface of the lamp.

BRIEF DESCRIPTION OF THE DRAWINGS In Figure 1, an embodiment of an anode 1 in a field emission light source according to the present invention is disclosed.

In Figure 2, a preferred embodiment of a field emission light source comprising the anode is disclosed.

In Figure 3, a graph showing the relation between the electrical conductance and the average thickness of the electrically conducting layer, based on experimental measurements, is disclosed.

In Figure 4, a graph showing the relation between the transparency and the average thickness of the

electrically conducting layer, based on experimental measurements, is disclosed.

DISCLOSURE OF PREFERRED EMBODIMENTS In Figure 1, an embodiment of an anode 1 in a field emission light source according to the present invention is disclosed. The anode 1 comprises an electrically conductive layer 3 and a luminescent layer 5 that is luminescent when excited by electron bombardment 7 caused by a potential difference 9 between the electrically conductive layer 3 and a cathode 11. The luminescent layer 5 is arranged between the electrically conductive layer 3 and the cathode 11. The electrically conductive layer 3 is a transparent electrically conducting layer 3.

The luminescent layer 5 is directly adjacent to the transparent electrically conductive layer 3. The thickness of the transparent electrically conducting layer 3 is in the interval 100-lOOOnm, more preferably in the interval 400-600nm and even more preferably in the interval 450-550nm. The potential difference 9, AV, is in the interval of 4-12 kV, and more preferably in the interval 5-llkV.

In one preferred embodiment the transparent electrically conducting layer is Indium Tin oxide (ITO), and in another preferred embodiment it is Zinc oxide (ZnO).

In the preferred embodiment, the anode 1 comprises an enclosing transparent structure 13 on which the electrically conducting layer 3 is fixed. In the preferred embodiment, the transparent structure 13 is a made of glass. In another preferred embodiment, the glass is provided with dimming features.

In Figure 2, a preferred embodiment of a field emission light source 15 comprising the anode 1 is disclosed. In Figure 2, an embodiment of an anode 1 in a field emission light source according to the present invention is disclosed. The anode 1 comprises an electrically conductive layer 3 and a luminescent layer 5 that is luminescent when excited by electron bombardment 7 caused by a potential difference 9 between the electrically conductive layer 3 and a cathode 11. The luminescent layer 5 is arranged between the electrically conductive layer 3 and the cathode 11. The electrically conductive layer 3 is a transparent electrically conducting layer 3. The luminescent layer 5 is directly adjacent to the transparent electrically conductive layer 3.

As can be seen in Figure 3, experiments performed indicates that the electrical conductance, S, increases as the average thickness of the electrically conducting layer 3 increases.

Further, as can be seen in Figure 4, experiments performed indicate that the transparency decreases as the average thickness of the electrically conducting layer 3 increases.

By interpreting the graphs in Figure 3 and Figure 4, in light of each other, it can be derived that an optimal thickness of the electrically conducting layer 3, is in the vicinity of 500nm. At the thickness of 500 nm, optical transparency of more than 95% is retained. This thickness is found sufficient for applications in relation to the present invention based on the consideration on the balance between the conductance, transparency and the time and economical aspects of the

process of applying the electrically conducting layer 3 on the enclosing transparent structure 13.

Description of measurements and experiments An ITO sol-gel solution was deposited on the inner surface of a glass tube, essentially making up the enclosing transparent structure 13, followed by baking at elevated temperatures. This creates ITO droplets. By repeating this procedure in a cyclic manner, for every cycle, the ITO droplets increasingly interconnect and as the average thickness of the electrically conducting layer 3 thus increases, the conductivity increases. The electrical conductance was measured after each cycle to monitor the transitions from near insulator to percolation conduction and to bulk conduction (as shown in Figure 3). The increased conductance reflects an improved connection between the droplets after each subsequent cycle, and that a substantial increase in conductance reflects the establishment of bulk conductivity. Further, an opacity tester was used after each cycle to monitor the transition from a transparent to an opaque condition (as shown in Figure 4).